1. Field of the Invention
The invention relates to gas turbine engines and, more particularly, to rotatable drums having radially inwardly extending blades for such engines.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor, and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some fan jet engines have been designed with counter-rotating turbines that power counter-rotating fans and boosters or low pressure compressors. U.S. Pat. Nos. 4,790,133, 4,860,537, 5,307,622 and 6,732,502 disclose counter-rotating low pressure turbines (LPT) that power counter-rotating fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan. There are also various designs for counter-rotating fan engines that use gearboxes to effect counter-rotation of the fans and boosters.
Counter-rotating boosters typically have interdigitated counter-rotating blades with a first set of booster blades counter-rotating with respect to a second set of booster blades wherein one of the sets of blades are mounted to and extend radially inwardly from a rotatable outer shell, drum or duct. A gas turbine engine booster having one or more rotatable booster stages with booster blades extending radially inwardly from a rotatable drum and one or more non-rotatable vane stages having booster vanes extending radially outwardly from a non-rotatable annular structure has also been developed as disclosed in U.S. patent application Ser. No. 12/275,813, currently designated as docket number 228194, entitled “AIRCRAFT GAS TURBINE ENGINE BOOSTER HAVING ROTATABLE RADIALLY INWARDLY EXTENDING BOOSTER BLADES AND NON-ROTATABLE RADIALLY OUTWARDLY EXTENDING BOOSTER VANES”, and assigned to the present Assignee, General Electric Company. Counter-rotating low pressure turbines have been developed that include a first annular low pressure drum with first turbine blade rows extending radially inwardly therefrom and axially spaced from each other. The first low pressure turbine blade rows are interdigitated with second low pressure turbine blade rows extending radially outwardly from a second annular low pressure drum.
The drums with radially inwardly extending blade rows have structural requirements that are hampered by the weight of the drum. The added weight of the drum structure necessary to mount the blades also tends to increase specific fuel consumption of the engine. The centrifugal force due to rotation of the blades must be countered by strengthening the drum by thickening and hence adding more weight to the drum. Thus, an alternative more robust drum design or configuration is desirable in order to minimize the weight of the drum.